1. Field of the Invention
This invention relates to a rocker arm of center pivot type or end pivot type, which is actuated by a cam in valve operation of an engine so as to open/close a valve provided on a cylinder head. In the rocker arm of the present invention, its major parts are produced by sheet metal processing.
2. Description of the Related Art
A conventional center pivot type rocker arm made of sheet metal has been disclosed in Japanese Unexamined Patent Application No.Sho58-152503.
This conventional rocker arm will be described. In this rocker arm, one ends thereof are free and the other ends are connected with each other. The opposing wall portions are separated from each other at a distance such that the inside walls opposes each other. The opposing wall portions of this pair are produced by folding one end of a single belt-like flat metal at 180.degree. relative to the other end at the center thereof in the length direction. Each of the opposing wall portions has a through hole at the middle point in the length direction of each wall portion. A cylindrical bushing is inserted through the through holes of the opposing wall portions and fixed there. A tilting fulcrum shaft is inserted into a hole of the bushing. The conventional rocker arm has such a structure. The tilting fulcrum shaft is supported by a cylinder head or the like.
In the aforementioned rocker arm bushing, one end thereof has a flange extending outward in the diameter direction so as to fit to an outside wall surface of one opposing wall portions. After this bushing is inserted into the through holes formed in the opposing wall portions of the pair, the other end of the bushing is caulked toward the outside wall surface of the other opposing wall portion.
In the conventional rocker arm, a load accompanied by the tilting motion of the tilting fulcrum shaft is applied to the through holes in the opposing wall portions through the bushing. Thus, an excessive bearing stress is applied to the through holes of the opposing wall portions. Therefore, this structure has a possibility that the opposing wall portions may be deformed with a passage of time so that a looseness occurs in the rocker arm.
Another conventional end pivot type rocker arm made of sheet metal has been disclosed in Japanese Unexamined Patent Application No.P63-272903.
In this rocker arm, one ends thereof are free and the other ends are connected with each other. The opposing wall portions are separated from each other at a distance such that the inside walls opposes each other. The opposing wall portions of this pair are produced by folding one end of a single belt-like flat metal at 180.degree. relative to the other end at the center thereof in the length direction.
A roller is rotatably supported at the middle of each of the opposing wall portions in the length direction in such a manner that a cam is in rolling contact with the roller. Chip members are attached to the free ends of the opposing wall portions so that they contact top end of a valve. The other ends of the opposing wall portions are curved in a cylindrical shape such that the curved portion is closed. To facilitate understanding, the cylindrical curve structure will be described more. This structure is the same as a structure formed by curving a single sheet metal in a cylindrical shape and then welding together both ends of the sheet metal. An internal surface of this cylindrical curved portion is threaded and a pivot shaft is fit to this cylindrical curved portion.
As described above, in the conventional rocker arm, the cylindrical curved portion of the other end of the opposing wall portions is formed by only bending the belt-like sheet metal and welding together both the opposing wall portions so as to form a closed structure. Thus, there is a high possibility that that cylindrical curved portion may be deformed by a twisting moment which repeatedly occurs accompanied by the tilting motion so that the connecting portion of the curved portion is peeled off.
To eliminate an occurrence of this peeling, there may be available a method in which a thickness of the opposing wall portions is increased so as to increase sectional area of the connecting portion thereby intensifying stiffness against a torsion moment. However, in this method, it is difficult to reduce the weight of the rocker arm and further achieve bending work of a high accuracy because of high stiffness.